Hi , I would like to try it

as u know in 1 minute pipe A and B can fill 1/20 and 1/24 respectively,and pipe C can empty 1/x ,in the question it's also given that all the three working at a time can fill the tank in 15 minutes.

We can frame an equation 1/20 +1/24 -1/x=1/15 ,after solving it we get x=40

pipe C can empty the tank at a rate of 3 gallons per minute so capacity of the tank is 40*3=120 gallons.

Thanks

It's all over to Daniel